RE: how many dpi to scan ?
I'm doing exactly what you are doing right now- using 8 1/2x11" 3 views to design a 70" warbird. Here's what I found works well: set your scan resolution to 1200 dpi and black and white. Hit the scan button and go get a cup of coffee! Creates a huge file, but so what- when you're done you'll delete it from you're drawing anyway? I've scaled some of the my scanned images in Autocad up by 50 to trace fine structural details, and the lines are still clear and thin.
Import the raster to one layer and trace on another so you can toggle them off and on to see how you're doing. Another tip: if you use splines, they look good but they're hard to edit. If you <save as> to a version R-12 file, splines get converted to polylines-- much eaiser to trim, etc.
John